The present disclosure relates to a light emitting device, and more particularly, to a substrate configured to improve light extraction efficiency of a light emitting device, and a light emitting device using the substrate.
The market of light emitting diodes (LEDs) has grown based on low-power LEDs used for keypads of portable communication devices such as cellular phones or small home appliances, and back light units of liquid crystal displays (LCDs). High-power, high-efficiency optical sources are recently required in the fields of interior lighting, exterior lighting, automobile interior or exterior lamps, back light units of large LCDs, etc., and thus the market of LEDs is now also concentrated on high-power products.
LEDs have a low light emitting efficiency. Generally, light emitting efficiency is determined by light generating efficiency (internal quantum efficiency), efficiency of guiding light outwardly (light extraction efficiency), and light conversion efficiency of a fluorescent material. Increasing the internal quantum efficiency by improving the characteristics of an active layer is effective to increase the output power of an LED; however, increasing the light extraction efficiency is more effective to increase the output power of an LED.
The biggest obstacle in guiding light to the outside of an LED may be internal total reflection caused by different refractive indexes of layers of the LED. Generally, due to different refractive indexes of layers of an LED, only about 20% of generated light can exit the LED. The rest of generated light is confined in the LED and is converted into heat as it moves in the LED. This results in a low light emitting efficiency and reduces the lifespan of the LED due to generation of heat.
Examples of light extraction efficiency increasing methods include a method of increasing the surface roughness of p-GaN or n-GaN, and a method of forming a rough or corrugated surface on a substrate which is a base of a light emitting device.
FIG. 1 is a sectional view illustrating a gallium nitride (GaN) LED 10 of the related art, and FIG. 2 is a perspective view illustrating a sapphire substrate 11. The GaN LED 10 includes the sapphire substrate 11 and a GaN light emitting structure 15 formed on the sapphire substrate 11.
The GaN light emitting structure 15 includes an n-type GaN cladding layer 15a, a multi-quantum well (MQW) active layer 15b, and a p-type GaN cladding layer 15c that are formed on the sapphire substrate 11. The GaN light emitting structure 15 may be grown by a process such as metal-organic chemical vapor deposition (MOCVD). Predetermined parts of the p-type GaN cladding layer 15c and the active layer 15b may be dry-etched to expose a topside part of the n-type GaN cladding layer 15a, and an n-type contact electrode 19 and a p-type contact electrode 17 may be formed on the exposed topside of the n-type GaN cladding layer 15a and the topside of the p-type GaN cladding layer 15c, respectively, so as to apply a voltage to the GaN LED 10. Generally, a transparent electrode 16 is formed on the topside of the p-type GaN cladding layer 15c before the p-type contact electrode 17 is formed, so as to increase a current injection area without reducing brightness.
The sapphire substrate 11 includes lenses 12 to improve light extraction efficiency. The lenses 12 used for the GaN LED 10 of the related art are generally hemisphere-shaped as shown in FIG. 2. Optimization of the shape and arrangement density of the lenses 12 is necessary to improve light extraction efficiency and characteristics of the GaN light emitting structure 15.